The invention relates to a display device comprising a matrix of picture electrodes on a substrate, which picture electrodes are connected via switching elements to drive electrodes for presenting drive signals.
Such active matrix displays are used in laptop and desktop computers, but also in, for example, TV and video applications.
For use of these displays, research has been done for some time on possibilities of using organic material. For example, a display device of the type mentioned above is known from U.S. Pat. No. 5,854,139. In the display device described in this patent, a separate structured layer of organic material for the switching element is realized for each individual picture electrode. To this end, it is necessary to deposit either the organic material via a mask with apertures at the locations of the transistors to be realized, or to provide a uniform layer of organic material and subsequently etch it. In both cases, an extra masking step is necessary. Moreover, etching of the organic semiconductor materials usually proceeds so fast that etching is not very well possible in a reproducible manner.
It is, inter alia, an object of the present invention to obviate the above-mentioned drawbacks as much as possible. To this end, a display device according to the invention is characterized in that the display device comprises a layer of organic material accommodating a plurality of switching elements.
According to the invention, the switching elements can be realized in one and the same layer of organic materials, with the switching elements being mutually insulated in at least the operating state. Since this layer does not need to be structured separately, a simpler manufacture is possible. Moreover, such layers can be easily provided on synthetic material substrates (plastic) so that the much heavier glass substrates are not necessary. The use of plastic substrates is possible because all process steps take place at temperatures up to approximately 150xc2x0 C. so that there are substantially no size changes.
A first embodiment of a display device according to the invention is characterized in that the substrate comprises a layer of organic semiconductor material at least at the area of the switching elements and the picture electrodes, and the display device comprises means for supplying at least one electrode, which, viewed perpendicularly to the substrate, substantially completely surrounds a picture electrode, with such a voltage that layers of organic semiconductor material at the area of switching elements and associated picture electrodes are substantially completely insulated from each other. The electrodes (guard lines) for a plurality of pixels are connected, for example, in an electrically conducting manner to an electrode which surrounds the complete picture surface and which is supplied with such a voltage during use that the semiconductor areas below (or above) the electrodes are insulating. The electrode may also be connected to a gate electrode of a TFT transistor in a matrix display. Since in a matrix of pixels only one row is selected at a time, the semiconductor areas below the gate electrodes are non-conducting during non-selection of the relevant row and constitute the desired insulator. By connecting the electrode to a gate electrode of a field effect transistor for a pixel in an adjacent row, a greater effective pixel surface is obtained.
If desired, said electrode may be provided as a pattern of guard lines underneath the semiconductor layer, with the pattern coinciding, for example, partly with that of the gate electrodes. In that case, the leakage currents are substantially completely eliminated. If necessary, said pattern may function as a black mask. The leakage currents may also be reduced by giving the gate electrode and said electrode meshing comb structures. Also the reverse structure (guard lines above and gate electrodes below the semiconductor layer) is possible.
Another embodiment of a display device according to the invention is characterized in that the layer of organic material comprises organic semiconductor material surrounded by insulating organic material at least at the area of the switching elements. In this case, the insulating organic material is obtained by selectively illuminating a polymer semiconductor material (for example, polythenylene vinylene (PTV)) locally with UV radiation by means of a mask so that it changes into insulating organic material. In this respect, it is to be noted that insulating the channel part for single TFT transistors is described in example 11 of U.S. Pat. No. 5,854,139.
Since depolarization may occur when using plastic substrates, polarizers are preferably used as substrates. It is alternatively possible to choose electro-optical effects at which no external polarizers are used such as, for example, PDLC.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.